ALSA: seq: Fix yet another races among ALSA timer accesses

ALSA sequencer may open/close and control ALSA timer instance
dynamically either via sequencer events or direct ioctls.  These are
done mostly asynchronously, and it may call still some timer action
like snd_timer_start() while another is calling snd_timer_close().
Since the instance gets removed by snd_timer_close(), it may lead to
a use-after-free.

This patch tries to address such a race by protecting each
snd_timer_*() call via the existing spinlock and also by avoiding the
access to timer during close call.

BugLink: http://lkml.kernel.org/r/CACT4Y+Z6RzW5MBr-HUdV-8zwg71WQfKTdPpYGvOeS7v4cyurNQ@mail.gmail.com
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/seq_timer.c

@@ -90,6 +90,9 @@ void snd_seq_timer_delete(struct snd_seq_timer **tmr)
 
 void snd_seq_timer_defaults(struct snd_seq_timer * tmr)
 {
+	unsigned long flags;
+
+	spin_lock_irqsave(&tmr->lock, flags);
 	/* setup defaults */
 	tmr->ppq = 96;		/* 96 PPQ */
 	tmr->tempo = 500000;	/* 120 BPM */
@@ -105,21 +108,25 @@ void snd_seq_timer_defaults(struct snd_seq_timer * tmr)
 	tmr->preferred_resolution = seq_default_timer_resolution;
 
 	tmr->skew = tmr->skew_base = SKEW_BASE;
+	spin_unlock_irqrestore(&tmr->lock, flags);
 }
 
-void snd_seq_timer_reset(struct snd_seq_timer * tmr)
+static void seq_timer_reset(struct snd_seq_timer *tmr)
 {
-	unsigned long flags;
-
-	spin_lock_irqsave(&tmr->lock, flags);
-
 	/* reset time & songposition */
 	tmr->cur_time.tv_sec = 0;
 	tmr->cur_time.tv_nsec = 0;
 
 	tmr->tick.cur_tick = 0;
 	tmr->tick.fraction = 0;
+}
+
+void snd_seq_timer_reset(struct snd_seq_timer *tmr)
+{
+	unsigned long flags;
 
+	spin_lock_irqsave(&tmr->lock, flags);
+	seq_timer_reset(tmr);
 	spin_unlock_irqrestore(&tmr->lock, flags);
 }
 
@@ -138,8 +145,11 @@ static void snd_seq_timer_interrupt(struct snd_timer_instance *timeri,
 	tmr = q->timer;
 	if (tmr == NULL)
 		return;
-	if (!tmr->running)
+	spin_lock_irqsave(&tmr->lock, flags);
+	if (!tmr->running) {
+		spin_unlock_irqrestore(&tmr->lock, flags);
 		return;
+	}
 
 	resolution *= ticks;
 	if (tmr->skew != tmr->skew_base) {
@@ -148,8 +158,6 @@ static void snd_seq_timer_interrupt(struct snd_timer_instance *timeri,
 			(((resolution & 0xffff) * tmr->skew) >> 16);
 	}
 
-	spin_lock_irqsave(&tmr->lock, flags);
-
 	/* update timer */
 	snd_seq_inc_time_nsec(&tmr->cur_time, resolution);
 
@@ -296,26 +304,30 @@ int snd_seq_timer_open(struct snd_seq_queue *q)
 	t->callback = snd_seq_timer_interrupt;
 	t->callback_data = q;
 	t->flags |= SNDRV_TIMER_IFLG_AUTO;
+	spin_lock_irq(&tmr->lock);
 	tmr->timeri = t;
+	spin_unlock_irq(&tmr->lock);
 	return 0;
 }
 
 int snd_seq_timer_close(struct snd_seq_queue *q)
 {
 	struct snd_seq_timer *tmr;
+	struct snd_timer_instance *t;
 	
 	tmr = q->timer;
 	if (snd_BUG_ON(!tmr))
 		return -EINVAL;
-	if (tmr->timeri) {
-		snd_timer_stop(tmr->timeri);
-		snd_timer_close(tmr->timeri);
-		tmr->timeri = NULL;
-	}
+	spin_lock_irq(&tmr->lock);
+	t = tmr->timeri;
+	tmr->timeri = NULL;
+	spin_unlock_irq(&tmr->lock);
+	if (t)
+		snd_timer_close(t);
 	return 0;
 }
 
-int snd_seq_timer_stop(struct snd_seq_timer * tmr)
+static int seq_timer_stop(struct snd_seq_timer *tmr)
 {
 	if (! tmr->timeri)
 		return -EINVAL;
@@ -326,6 +338,17 @@ int snd_seq_timer_stop(struct snd_seq_timer * tmr)
 	return 0;
 }
 
+int snd_seq_timer_stop(struct snd_seq_timer *tmr)
+{
+	unsigned long flags;
+	int err;
+
+	spin_lock_irqsave(&tmr->lock, flags);
+	err = seq_timer_stop(tmr);
+	spin_unlock_irqrestore(&tmr->lock, flags);
+	return err;
+}
+
 static int initialize_timer(struct snd_seq_timer *tmr)
 {
 	struct snd_timer *t;
@@ -358,13 +381,13 @@ static int initialize_timer(struct snd_seq_timer *tmr)
 	return 0;
 }
 
-int snd_seq_timer_start(struct snd_seq_timer * tmr)
+static int seq_timer_start(struct snd_seq_timer *tmr)
 {
 	if (! tmr->timeri)
 		return -EINVAL;
 	if (tmr->running)
-		snd_seq_timer_stop(tmr);
-	snd_seq_timer_reset(tmr);
+		seq_timer_stop(tmr);
+	seq_timer_reset(tmr);
 	if (initialize_timer(tmr) < 0)
 		return -EINVAL;
 	snd_timer_start(tmr->timeri, tmr->ticks);
@@ -373,14 +396,25 @@ int snd_seq_timer_start(struct snd_seq_timer * tmr)
 	return 0;
 }
 
-int snd_seq_timer_continue(struct snd_seq_timer * tmr)
+int snd_seq_timer_start(struct snd_seq_timer *tmr)
+{
+	unsigned long flags;
+	int err;
+
+	spin_lock_irqsave(&tmr->lock, flags);
+	err = seq_timer_start(tmr);
+	spin_unlock_irqrestore(&tmr->lock, flags);
+	return err;
+}
+
+static int seq_timer_continue(struct snd_seq_timer *tmr)
 {
 	if (! tmr->timeri)
 		return -EINVAL;
 	if (tmr->running)
 		return -EBUSY;
 	if (! tmr->initialized) {
-		snd_seq_timer_reset(tmr);
+		seq_timer_reset(tmr);
 		if (initialize_timer(tmr) < 0)
 			return -EINVAL;
 	}
@@ -390,11 +424,24 @@ int snd_seq_timer_continue(struct snd_seq_timer * tmr)
 	return 0;
 }
 
+int snd_seq_timer_continue(struct snd_seq_timer *tmr)
+{
+	unsigned long flags;
+	int err;
+
+	spin_lock_irqsave(&tmr->lock, flags);
+	err = seq_timer_continue(tmr);
+	spin_unlock_irqrestore(&tmr->lock, flags);
+	return err;
+}
+
 /* return current 'real' time. use timeofday() to get better granularity. */
 snd_seq_real_time_t snd_seq_timer_get_cur_time(struct snd_seq_timer *tmr)
 {
 	snd_seq_real_time_t cur_time;
+	unsigned long flags;
 
+	spin_lock_irqsave(&tmr->lock, flags);
 	cur_time = tmr->cur_time;
 	if (tmr->running) { 
 		struct timeval tm;
@@ -410,7 +457,7 @@ snd_seq_real_time_t snd_seq_timer_get_cur_time(struct snd_seq_timer *tmr)
 		}
 		snd_seq_sanity_real_time(&cur_time);
 	}
-                
+	spin_unlock_irqrestore(&tmr->lock, flags);
 	return cur_time;	
 }